Charge neutralization during high-current ion implantation
During ion implantation, positive charge accumulates due to excess positive beam ions incident on the wafer as well as through the emission of secondary electrons from the wafer surface. This positive charge build-up is counteracted by the flow of electrons onto the wafer surface. These electrons are drawn from the beam plasma, from supplemental sources of electrons ("flood guns"), and from current flows along the wafer surface and through the bulk . In a typical situation, these charge flows are not exactly balanced, especially in local regions (such as on isolated polysilicon structures), leading to voltage build-up and degradation or destruction of gate oxides and other dielectric films.
A successful charge neutralization technology strives to balance the total charged flux into the region near the wafer surface and minimize net charge flow to the wafer. The potential differences between the ion beam plasma and the structures on the wafer surface influence the charge flow to the surface and must be minimized. Since build-up of excess electrons on the wafer surface may also lead to damage in sensitive dielectric films ("overflooding"), the energy of the electrons that are used to balance the positive charge flow must be lower than those potentials that can lead to oxide wearout and breakdown problems.
In ion implanters, the beam generates too few electrons by collisional ionization of background gases to balance the positive ion beam current. To neutralize the beam, an auxiliary source of electrons--an electron "flood gun"--must provide additional negative charge. Over the past decade, a wide variety of electron flood guns have been incorporated into nearly all types of high-current ([is greater than]2-mA) implanters and many types of medium-current systems .
Electron flood gun techniques, which generate secondary electrons by electron bombardment of metal surfaces or by impact ionization of inert gases, are widely used. However, these electron sources produce electrons of such high energy that they build up damaging negative potentials and fail to compensate excess charges in microscopic structures. Changes in the wafer conditions during implantation, such as outgassing and carbonization of photoresist layers, demand close monitoring and tuning of the electron shower in order to achieve charge compensation throughout the implant cycle .
Low-energy electrons provide a better solution to the surface charge neutralization problem. In addition to limiting the negative potentials that can result from excess electron flows, low-energy electrons can respond to small potential changes in the surface, compensating charge imbalances in microscopic regions on the IC device and reducing the variation in sensitivity to charging damage of individual device designs. Such electrons can be added to the ion beam plasma using the "plasma-bridge" concept.
Conventional ion beam plasma
The beam plasma consists of high-energy "beam" ions (accelerated by the beamline potentials), low-energy "slow" ions (created by collisions between the beam ions and background gas atoms), and electrons from various sources. In a conventional space-charge-neutralized beam, the density of electrons approximately equals the sum of the densities of the beam ions and slow ions, so that the beam...
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